In making shaped metal articles, metal castings are typically made by pouring molten metal into molds which may be made of sand bonded with various types of organic or inorganic binders. When the casting is removed from the sand mold, the mold is disintegrated and cannot be reused, except to reclaim the sand for future molding operations. Molds may also be of a permanent type. Permanent molds are made of solid materials such as metal or graphite and often consist of two or more pieces that can be separated to remove the casting. After the casting is removed, the mold is reused.
Internal cavities within a metal casting are made by placing cores inside a mold before the metal is cast. The core must then be removed from the interior of the casting to leave the cavity thus formed.
Cores for metal castings generally are made with a particulate refractory material such as sand bonded with an inorganic or, more commonly, an organic binder. The organic binder must serve to maintain the physical integrity of the core until the metal solidifies but then must sufficiently decompose, due to the heat from the casting operation, to allow the removal of the sand from the casting. The removal of core sand is typically effected by a process called shakeout which involves mechanically vibrating and impacting a casting to free the sand, which then may be reclaimed and reused for subsequent casting operations.
Many different types of organic binders are used to make sand cores for metal castings and many different processes used to cure these binders. One type of binder consists primarily of a phenolic resole resin. Phenolic resoles are typically made by reacting 1.0 to 3.0 moles of formaldehyde with one mole of a phenolic compound using an alkaline catalyst. Such binders are classified as "thermosetting" in that heat alone will cure them, but can also be cured with acid catalysts either at room temperature or with the help of heat. Foundry sand core binders based on phenolic resins are known which are cured by the above processes.
In recent years, ambient temperature ester-curable alkaline phenolic resole resins have gone into widespread use. Such resin binder systems are disclosed, for example, in U.S. Pat. Nos. 4,426,467 and 4,474,904, in which lactones and carboxylic acid esters, respectively, are used as curing agents; and in 4,468,359, in which the esters are used in the gaseous or vapor phase.
One use for which these resins are eminently suited is as binders for making foundry sand molds and cores. They display high casting quality and hardness; are rapidly cured at ambient temperature; and they do not evolve pungent gases on their thermal decomposition. However, such resins have one inherent disadvantage in that, when used to make cores for some types of castings, these cores exhibit poor shakeout relative to some other types of organic binders. Such poor shakeout can occur when casting metals having low melting points are used, or when a large core size relative to the amount of metal being poured is used. Primarily, however, poor shakeout results when binders fail to decompose sufficiently after casting to allow the sand to be easily removed.
Reduced hot strength and enhanced collapsibility are also desirable properties for molding cores in that such properties help prevent a casting defect known as hot tearing: when molten metal is poured around a core, the metal begins to shrink as it solidifies and cools. In order to prevent the hot, shrinking casting from tearing, the core must be able to collapse to some extent; if it does not, hot tearing may result. The present invention also seeks to reduce this defect.
Thus, a method to reduce hot strength of binders used in foundry cores and to improve the shakeout characteristics of phenolic resole based sand binders would expand the market for these binders and be very desirable.